This invention relates to electrical motors of the stepping type and more particularly to a stepping motor utilizing permanent magnet excitation. The rotor is caused to advance in steps controlled by the relative spacing of the teeth on the rotor and on the poles of the stator. As the stator poles are successively energized by associated windings, the rotor moves so as to bring its teeth into registration with the teeth on the stator pole faces.
Digital information can be transformed into mechanical motion in the stepping motor where the rotor's movement through defined angles is a function of the pulses applied to the stator coils. For many applications, it is essential that the stepping motor be rapidly started and stopped. One such application is the drive of the paper feed in a high speed printer for data processing equipment where the paper is to be moved from one line to another. Since the mass of the paper to be transported is small, acceleration is essentially a function of the motor's moment of inertia.
There has been continuing effort directed towards the development of motors having a high torque-to-inertia ratio and therefore, better acceleration and deceleration. There have been brush and collector motors having hollow frusto-conical coil carrying rotors. The frusto conical shape provides enhanced mechanical braking action or ready adjustment of the air gap between rotor and stator. The low inertia of these motors was primarily attributable to the hollowness of the rotor structure and the lightness of the synthetic material used. In order to improve the torque-to-inertia ratio of stepping motors, various techniques have been used to reduce the mass of the rotor which is usually cylindrically shaped. A reduction in rotor diameter reduces inertia but also has the disadvantage of reducing torque. Hollow cylindrical rotors have a torque-to-inertia ratio improved by about a factor 2 as a result of the decrease in mass without a corresponding decrease in torque.
It is the primary object of this invention to further improve the torque-to-inertia ratio of stepping motors thereby providing better acceleration and decreased rotor oscillation.